New electronic equipments such as portable telephones and notebook type personal computers have been recently developed one after another. With strong demands for miniaturization, weight-lightening and handy carrying of these merchandise, microcomputers, IC memories and other parts incorporated in the electronic equipments have been required to be miniaturized and to have high performance.
The elements for constituting high-performance miniaturized microcomputers, such as CPU, or the IC memories, however, undergo stall and memory erasure by the mere instantaneous discontinuance or decrease of power supply, whereby the electronic equipments occasionally misfunction. In fact, it is known that the electronic equipments misfunction by the mere voltage drop of 10 to 20% for 0.003 to 0.02 sec, unless proper measures are taken.
In order to prevent the discontinuance or decrease of power supply, Ni--Cd batteries or aluminum electrolytic capacitors are currently used for back-up power sources of the microcomputers and the IC memories. However, these power sources are not sufficient from the viewpoints of working temperature range, number of charge and discharge cycles, capacity, quick charge and discharge properties and cost.
In the circumstances, electric double layer capacitors have been recently paid much attention as the back-up power sources.
When two kinds of materials different in physical properties are contacted with each other, positive and negative electric charges are generally arrayed at short invervals with interposing the interface therebetween. The electric charge distribution formed on the interface is called an electric double layer. The electric double layer capacitor is a kind of a condenser to form an electric double layer of large capacity on the interface between an electrode having a large surface and an electrolytic solution and to release the electric charges of the electric double layer.
Studies of the electric double layers have history which can trace away back to the past, on the time of the study of Helmholtz in 1879. However, a long period of time was necessary for the practical use of the electric double layer capacitors, and a capacitor having large capacity of farad unit using this principle was obtained only in the early 1980's. In the present-day electric double layer capacitors, an organic solvent type electrolytic solution or an aqueous solution type electrolytic solution is used as the electrolytic solution, and granular, massive or fibrous activated carbon having large specific surface area is used as the electrode material, whereby large capacity is realized.
The electric double layer capacitors are not associated with such a chemical reaction as occurs in the conventional secondary batteries during the charge and discharge operations, so that their internal resistance is exceptionally low as compared with that of the secondary batteries, and besides large current discharge and quick charge and discharge are feasible. Moreover, the capacitors are free from deterioration caused by charge and discharge cycles and limitation on the number of charge and discharge cycles.
Because of these advantages, there have been proposed in recent years uses of the electric double layer capacitors not only in the fields concerned to small electric power such as a field of the aforesaid portable electronic equipments but also in the fields concerned to large capacity such as a field of auxiliary power sources of automobile batteries. For example, electric automobiles and gasoline-fueled automobiles, which are mounted with an electric double layer capacitor to charge a part of regenerative kinetic energy generated in the deceleration and to discharge the energy in the acceleration so as to supplement engine output energy, are manufactured by way of trial.
The most serious problem of the electric double layer capacitors is that the discharge capacity is smaller than that of the secondary batteries. To cope with this problem, various researches on the electrolytic solutions and the electrode materials have been made now.
For example, the activated carbons used as the electrode materials of the electric double layer capacitors are manufactured by the use of so called hard carbon (non-graphitizable carbon) materials, such as those made from coconut shell, coal and phenol resin, as starting materials. The non-graphitizable carbon materials are generally activated with water vapor or water vapor-containing exhaust gas generated by combustion of propane, kerosene, etc. to give activated carbons. In this activation treatment, pores are formed by a carbon elimination caused by the reaction of water vapor and/or carbon dioxide with carbons, and the pores define a density of fiber and a specific surface area of the activated carbons.
In general, the discharge capacity per unit weight of the electrode used for the electric double layer is proportional to the specific surface area of the activated carbon, and in the convention activation treatment, extremely severe conditions must be adopted to obtain activated carbon having a large specific area.
For example, to manufacture an electric double layer capacitor having large discharge capacity, an electrode formed from activated carbon having a specific surface area of not less than 2,000 m.sup.2 /g (measured value by BET method) is desired to be used, and the activated carbon having such a large specific surface area as above should be prepared by activating a carbon material, e.g., hard carbon, under such severe activation conditions that the activation yield is reduced to about 10 to 20% by weight or lower.
More specifically, phenol resin-based activated carbon fibers obtained by activating phenol resin based carbon fibers in a combustion gas such as propane have a large specific surface area such as 2,500 m.sup.2 /g and are commercially used for electric double layer capacitors, but the activation yield in this activation treatment is as extremely low as about 15% by weight.
The phenol resin-based activated carbon fibers exhibit excellent properties in water treatment or adsorption of harmful gas, but when they are used as electrode materials of electric double layer capacitors, they cannot raise electrode density because of low density of fiber, and hence the charge and discharge capacity per unit volume is not increased so much.
The phenol resin-based activated carbon fibers have another problem in that even if the specific surface area measured by BET method is enlarged, the discharge capacity per unit weight is not increased in proportion to the increase of specific surface area. It is presumably indicated by the phenomenon that not all of the surfaces of the activated carbon fibers obtained by the conventional activation method are utilized for the formation of the electric double layer. That is, it is presumed that, in order to form the electric double layer, activated carbon should have pores of optimum diameters for the electrolytic solution used, but the pores of the activated carbon formed by the conventional activation treatment are not optimum for the electric double layer capacitor.
As described above, the density of the conventional activated carbon is inevitably lowered by adoption of the severe activation conditions. Besides, pores having diameters suitable for the electrolyte solution cannot be sufficiently formed and, therefore, the surface area effective for the formation of an electric double layer is small. For these reasons, any activated carbon capable of realizing electrodes of electric double layer capacitors having sufficiently large discharge capacity per electrode unit volume and per electrode unit weight has not been developed in the existing circumstances.
For example, according to the measurement by the inventors, an electric double layer capacitor, which uses an electrode comprising phenol resin-based activated carbon fibers obtained by the conventional activation method and uses an organic solvent type electrolytic solution, has a discharge capacity of about 20 F/g which is markedly lower than that of the later-described examples of the invention.
In the conventional activation treatment wherein the non-graphitizable carbon material is activated with water vapor or exhaust gas, further, the material needs to be activated under severe conditions in order to obtain activated carbon having large specific area capable of improving the electrical capacity per unit weight, and such activation treatment causes increase of cost of the activated carbon.
The present inventors have earnestly studied to solve such problems associated with the prior art as described above and has found as a result that activated carbon fibers having excellent conductivity, high density, large surface area and, when applied for an electrode of an electric double layer, capable of realizing high discharge capacity can be obtained by activating a soft carbon (easily graphitizable carbon) type material, particularly an infusibilized mesophase pitch-based carbon fibers, after a specific treatment.
The inventors have further found from the observation and comparison of many kinds of mesophase pitch-based activated carbon fibers and non-mesophase pitch-based activated carbon fibers obtained by the exhaust gas activation having various BET specific surface areas that the high discharge capacity is dependent not only on the specific surface area, but also on the distribution state (by volume) of pores having specific radii in the activated carbon fibers.
Based on the finding, the present invention has been accomplished.
As other activation treatment methods than the above-mentioned ones using water vapor or exhaust gas, heat treatment methods using alkali metal compounds are disclosed in U.S. Pat. No. 3,817,874 and No. 4,082,694.
However, any trial for using mesophase pitch-based active carbon fibers as materials for electrodes of electric double layer capacitors had not been made at all.
Japanese Laid-Open Publication No. 1(1989)-139865 discloses a process comprising activating carbon fibers in the presence of an alkali metal compound and Japanese Laid-Open Publication No. 5(1993)-247731 discloses a process for producing carbon fibers having high specific surface area, in which the above technique is applied to a carbon fibers of mesophase of not less than 50%.
However, these documents never disclose or suggest the use of the activated carbon fibers for the double layer capacitor, a process for preparing, from graphitizable carbon material, activated carbon fibers having a pore distribution suitable for forming an electric double layer, and the pore distribution.